Nonlinear class AB input stage

ABSTRACT

A circuit includes a differential input stage amplifier that receives a differential input voltage and generates an output voltage based on a difference in the differential input voltage. A feedback loop provides feedback from an output of the differential input stage amplifier to input tail current of the differential input stage amplifier. The feedback loop enables class AB operation of the differential input stage amplifier. At least one gain reducer is operatively coupled to the feedback loop to reduce the gain of the feedback loop. The gain reducer has a resistance value that varies inversely proportional to loop current in the feedback loop to reduce the gain of the feedback loop as loop current increases.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 61/938,796 filed on 12 Feb. 2014, and entitled CLASS ABINPUT STAGE FOR LDO'S AND AMPLIFIERS, the entirety of which isincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to integrated circuits, and more particularly toa gain reduction and feedback circuit to facilitate stabilization of anonlinear class AB input amplifier stage.

BACKGROUND

Class AB amplifiers provide a suitable compromise between linear class Aoperation with higher amplifier power dissipation and non-linear class Boperation. One application for class AB amplifiers is with a low dropoutlinear regulator circuits which are also referred to as LDO's. Inexisting stand-alone LDO's, the output dynamic error should be keptwithin 150-200 mV when instantly switching load current from a fewmicroamps to 100-150 mA, for example, which represents a large dynamicrange. It is also desirable to keep LDO quiescent current (Iq), which isdominated by an error amplifier in the LDO, in the range of 100-200 nAif possible. To provide low Iq and dynamic error while controlling anoutput pass device of the LDO (e.g., pass device requires large currentfor dynamic charging/discharging of the gate capacitance of the passdevice during load transient), 1000 times or more increase in tailcurrent at 100-150 mV input error is generally required. Tail current isthe current supplied to the input differential stage amplifier of theerror amplifier in response to a differential voltage input. To achieve1000 times increase in tail current, input error for the differentialinput stage should be on order of 500-600 mV. Some circuits require aminimum input voltage and provide more gain in the feedback loop toyield a somewhat sharper tail current boost—for example, 300-400 mV ofinput error yields 1000 times increase in tail current. Nonlinearoperation provided by 300-400 mv input differential is still notsufficient however. To decrease the input differential even more, largerfeedback loop gain may be required. Providing larger gain in thefeedback loop can decrease amplifier stability however at some points ofsuch a large dynamic range of load current.

SUMMARY

This disclosure relates to a gain reduction and feedback circuit tofacilitate stabilization of a nonlinear class AB input amplifier stage.In one example, a circuit includes a differential input stage amplifierthat receives a differential input voltage and generates an outputvoltage based on a difference in the differential input voltage. Afeedback loop provides feedback from an output of the differential inputstage amplifier to input tail current of the differential input stageamplifier. The feedback loop enables class AB operation of thedifferential input stage amplifier. At least one gain reducer isoperatively coupled to the feedback loop to reduce the gain of thefeedback loop. The gain reducer has a resistance value that variesinversely proportional to loop current in the feedback loop to reducethe gain of the feedback loop as loop current increases.

In another example, a circuit includes a differential input stageamplifier that receives a differential input voltage and generates anoutput voltage based on a difference in the differential input voltage.A negative feedback loop provides feedback from an output of thedifferential input stage amplifier to input tail current of thedifferential input stage amplifier. The negative feedback loop enablesclass AB operation of the differential input stage amplifier. A positivefeedback loop provides feedback from an output of the differential inputstage amplifier to input tail current of the differential input stageamplifier. The positive feedback loop reduces an amount of differentialinput voltage to initiate nonlinear operations of the differential inputstage amplifier with respect to changes in the differential inputvoltage. At least one gain reducer is operatively coupled to thenegative feedback loop or the positive feedback loop to reduce the gainof the negative feedback loop or the positive feedback looprespectively. The gain reducer has a resistance value that variesinversely proportional to loop current in the negative feedback loop orthe positive feedback loop to reduce the gain of the respective loop asloop current increases.

In yet another example, a circuit includes a differential input stageamplifier that receives a differential input voltage and generates anoutput voltage based on a difference in the differential input voltage.A negative feedback loop provides feedback from an output of thedifferential input stage amplifier to input tail current of thedifferential input stage amplifier. The negative feedback loop enablesclass AB operation of the differential input stage amplifier. A positivefeedback loop provides feedback from an output of the differential inputstage amplifier to input tail current of the differential input stageamplifier. The positive feedback loop reduces an amount of differentialinput voltage to initiate nonlinear operations of the differential inputstage amplifier with respect to changes in the differential inputvoltage. At least one gain reducer is operatively coupled to thenegative feedback loop and the positive feedback loop to reduce the gainof the negative feedback loop and the positive feedback loop. The gainreducer has a resistance value that varies inversely proportional toloop current in the negative feedback loop and the positive feedbackloop to reduce the gain of the respective loop as loop currentincreases. An output driver stage amplifies the output voltage from thedifferential input stage amplifier and generates an amplified outputvoltage to provide an operational amplifier output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a class AB amplifier circuit thatemploys feedback loops and gain reducers to enhance stability andnonlinear operations of the circuit.

FIG. 2 illustrates an example of low dropout regulator having a class ABamplifier circuit that employs feedback loops and gain reducers toenhance stability and nonlinear operations of the circuit.

FIG. 3 illustrates an example of an operational amplifier having a classAB amplifier circuit that employs feedback loops and gain reducers toenhance stability and nonlinear operations of the circuit.

FIG. 4 illustrates a particular example of a class AB amplifier circuitutilized for a low dropout linear regulator application that employsfeedback loops and gain reducers to enhance stability and nonlinearoperations of the circuit.

FIG. 5 illustrates an alternative example of a class AB amplifiercircuit configured as an operational amplifier that employs feedbackloops and gain reducers to enhance stability and nonlinear operations ofthe circuit.

DETAILED DESCRIPTION

This disclosure relates to a gain reduction and feedback circuit tofacilitate stabilization and tail current boost of a nonlinear class ABinput amplifier stage. The class AB input amplifier stage includes adifferential input stage amplifier that receives a differential inputvoltage and generates an output voltage based on a difference in thedifferential input voltage. In order to provide the most efficientoperation of the stage, it is desirable to have sharp output transitionat the lowest possible difference between the differential inputvoltages. To achieve this, large increases in tail current for thedifferential input stage are required in response to a minimum inputvoltage differential. In conventional circuits, negative feedback loopgain for the differential input stage is increased. By increasing loopgain of the feedback loop to provide such functionality however, leadsto stability problems over the large dynamic range of load currents thatare generated. To facilitate stability over the large dynamic range, again reducer (or reducers) can be coupled to the feedback loop in thedifferential input stage to dynamically reduce the gain of the feedbackloop over the dynamic range. The gain reducer has a resistance value(e.g., 1/gm, where gm is a transconductance parameter) that variesinversely proportional to loop current in the feedback loop to reducethe gain of the feedback loop as loop current increases (e.g., thehigher the loop current the lower the resistance value). In this manner,the loop can be stabilized. In another aspect, a positive feedback loopcan be provided in addition to the negative feedback loop to facilitatenonlinear operation of the differential input stage. This includesboosting tail current with a lower amount of input voltage differential(e.g., providing 1000 times tail current boost with 100-120 mVdifferential input voltage as opposed to conventional circuit of 300-400mV or larger).

FIG. 1 illustrates an example of a class AB amplifier circuit 100 thatemploys feedback loops and gain reducers to enhance stability andnonlinear operations of the circuit. As used herein, the term circuitcan include a collection of active and/or passive elements that performa circuit function such as an analog circuit or control circuit, forexample. The term circuit can also include an integrated circuit whereall the circuit elements are fabricated on a common substrate, forexample. A differential input stage amplifier 110 receives adifferential input voltage shown as VINP and VINN (where P and Nrepresent positive and negative) and generates an output voltage VOUTbased on a difference in the differential input voltage. As will beshown in more detail with respect to FIGS. 4 and 5, the differentialinput stage amplifier 110 includes a differential pair of transistorsthat receives VINP and VINP, where the differential pair with theaddition of negative and positive feedback loop adjusts its own tailcurrent in response to a voltage difference between VINP and VINN. Asused herein the term tail current refers to the combined currents in thesource terminals of the differential pair of transistors in thedifferential input stage amplifier 110.

For efficient nonlinear class AB operations, it is desirable to generatethe largest amount of tail current with the smallest amount of voltagedifferential between VINP and VINN. To achieve this, feedback loops 120are provided. A negative feedback loop 130 provides feedback from anoutput of the differential input stage amplifier 110 to tail currentinput of the differential input stage amplifier. The negative feedbackloop 130 enables class AB operation of the differential input stageamplifier 110. A positive feedback loop 140 provides feedback from anoutput of the differential input stage amplifier 110 to tail currentinput of the differential input stage amplifier. The positive feedbackloop 140 reduces an amount of differential input voltage to initiatenonlinear operations of the differential input stage amplifier 110 withrespect to changes in the differential input voltage VINP and VINN. Forexample by providing positive feedback to the feedback loops 120,conventional voltage differentials of 300-400 mV to generate 1000 timesor more increase in tail current can be reduced to a range from about100 to about 120 millivolts (mV), for example. The gain of the positivefeedback loop 140 is typically set substantially smaller than that ofthe negative feedback loop. For example, the positive feedback loop gainmay be one fourth (or less) than the gain set for the negative feedbackloop.

To facilitate stability over a large dynamic range of load currents thatare supported, one or more gain reducers shown as gain reducer 1 thoughN, where N is a positive integer, can be coupled to the feedback loop120 in the differential input stage amplifier 110 to dynamically reducethe gain of the feedback loop over the dynamic range of load currentssupported. The gain reducers 1 through N can be employed in either thenegative feedback loop 130 and/or the positive feedback loop 140. Thegain reducer has a resistance value (e.g., 1/gm) that varies inverselyproportional to loop current in the feedback loop 120 to reduce the gainof the feedback loop as loop current increases (e.g., the higher theloop current the lower the resistance value). In this manner, thefeedback loop 120 (or loops) can be stabilized. In one example, the gainreducer can be a diode and/or a transistor configured as a diode. Thegain reducer can be placed in series with an amplifier stage to limitits current where its resistance can vary as loop current varies. Inconventional circuits, fixed resistors were employed in place of thegain reducers where the fixed resistors had a constant and unvaryingresistance as opposed to resistance to the gain reducer which reducesresistance with increasing current. As such, as load current increasedover a large dynamic range in conventional circuits, instability couldoccur (e.g., ringing in the output) at larger increases in loop gain.

FIG. 2 illustrates an example of low dropout regulator 200 having aclass AB amplifier circuit 204 that employs feedback loops and gainreducers to enhance stability and nonlinear operations of the circuit.In this example, the class AB amplifier circuit 204 is employed as anerror amplifier in the low dropout regulator 200. The LDO linearregulator 200 (also referred to as LDO) generates a regulated outputvoltage VOUT and a load current IL. The LDO 200 includes the class ABamplifier 204 configured as an error amplifier having a reference inputthat receives a reference voltage VREF and a load feedback input thatreceives feedback LOAD FB from the regulated output voltage. The classAB error amplifier 204 generates an error output signal ERR OUT tocontrol the regulated output voltage VOUT of the LDO linear regulator200 based on a voltage difference between the load feedback input andthe reference input. A pass device 208 has an input that receives theerror output signal from the class AB error amplifier 204 and switches(with variable ON resistance) an input voltage VIN to the regulatedoutput voltage of the LDO linear regulator based on the error outputsignal.

Similar to FIG. 1 above, the class AB amplifier 204 includes adifferential input stage amplifier 210 receives a differential inputvoltage shown as VREF and LOADFB and generates an output voltage VOUTbased on a difference in the differential input voltage. Feedback loops220 are provided and can include a negative feedback loop 230 thatprovides feedback from an output of the differential input stageamplifier 210 to an input of the differential input stage amplifier. Thenegative feedback loop 230 enables class AB operation of thedifferential input stage amplifier 210. A positive feedback loop 240provides feedback from an output of the differential input stageamplifier 210 to an input of the differential input stage amplifier. Thepositive feedback loop 240 reduces an amount of differential inputvoltage to initiate nonlinear operations of the differential input stageamplifier 210 with respect to changes in the differential input voltageVREF and LOADFB. One or more gain reducers shown as gain reducer 1though N, where N is a positive integer, can be coupled to the feedbackloop 220 in the differential input stage amplifier 210 to dynamicallyreduce the gain of the feedback loop over the dynamic range of loadcurrents supported. The gain reducers 1 through N can be employed ineither the negative feedback loop 230 and/or the positive feedback loop240.

FIG. 3 illustrates an example of an operational amplifier 300 having aclass AB amplifier 304 circuit that employs feedback loops and gainreducers to enhance stability and nonlinear operations of the circuit.In this example, the operational amplifier 300 includes an output driverstage 308 to amplify an output voltage from a differential input stageamplifier 310 and generate an amplified output voltage VOUT to providean operational amplifier output. The output driver stage 308 can includea complimentary pair of transistors as will be illustrated and describedbelow with respect to FIG. 5.

Similar to FIG. 1 and above, the class AB amplifier 304 includes thedifferential input stage amplifier 310 and receives a differential inputvoltage shown as VINP and VINN and generates an output voltage to theoutput driver stage 308 based on a difference in the differential inputvoltage. Feedback loops 320 are provided and can include a negativefeedback loop 330 that provides feedback from an output of thedifferential input stage amplifier 310 to an input of the differentialinput stage amplifier. A positive feedback loop 340 provides feedbackfrom an output of the differential input stage amplifier 310 to an inputof the differential input stage amplifier. One or more gain reducersshown as gain reducer 1 though N, where N is a positive integer, can becoupled to the feedback loop 320 in the differential input stageamplifier 310 to dynamically reduce the gain of the feedback loop overthe dynamic range of load currents supported. The gain reducers 1through N can be employed in either the negative feedback loop 330and/or the positive feedback loop 340.

FIG. 4 illustrates a particular example of a class AB amplifier circuit400 that employs feedback loops and gain reducers to enhance stabilityand nonlinear operations of the circuit. The circuit 400 includescurrent sources I1 and I2. I2 drives M6 which supplies tail currentcollectively to transistors M0 and M1 which represent the differentialamplifier previously described. Output from M0 and M1 can be utilized todrive a pass device such as described above with respect to FIG. 2, forexample. Transistors M0A and M1A are placed in parallel to M0 and M1 tocarry proportional sampled currents as in M0 and M1 and these receivereference input voltage VREF and feedback from the load shown as VOUT.Differences between VREF and VOUT cause the amplifier circuit 400 toleave its quiescent state. Output from M0A and M1A are connected to thesource of M8 and M9 respectively. In this example, M7, M10, M11, andM14, operate as gain reducers that were previously described. The gainreducers are transistors configured as diode in these examples butregular diodes could also be employed. Transistors M12 and M13 operatein conjunction with the respective feedback loops described herein toprovide positive feedback. A negative feedback loop is formed via M8,M0A, and M6. A positive feedback loop is formed via transistors M1A,M13, M8, and M6, for example.

As noted previously, the stability in class AB can be facilitated bydecreasing gain by using diodes instead of resistors. At the same time,low-gain positive feedback M12 and M13 increase tail current boost atsmaller input voltages differential to this differential pair. Gain ofpositive feedback is 1/N, where N is ratio of W/L's (width/length)between M11 and M12 and M13 and M14. With 1/N=0.25 for example, 1000×tail current boost is achieved at 100-120 mV input errors which is overa two times improvement of 300-400 mV conventional circuits.

FIG. 5 illustrates an alternative example of a class AB amplifiercircuit 500 that employs feedback loops and gain reducers to enhancestability and nonlinear operations of the circuit. For purposes ofbrevity, each circuit component of the circuit 500 will not be describedbut rather the difference with respect to the circuit 400 describedabove. In this example, outputs from differential amplifier formed fromM0 and M1 are tied internally to the source of M8 and M9 respectively.Rather than being driven in parallel to M0A and M1A as describedpreviously, M0 and M1 are driven directly from operational amplifierinputs VINN and VINP. An output drive stage is provided that includesM16 driven from M9 (through M14) and M18 driven from M8 (through M11,M15 and M17). Gain reducer M17 is also included in the output driverstage. As shown, M16 and M18 are configured as a complimentary pair (oneNPN and one PNP) with their respective drains tied to provide an outputvoltage VOUT.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

What is claimed is:
 1. A circuit comprising: a differential input stageamplifier that receives a differential input voltage and generates anoutput voltage based on a difference in the differential input voltage;a feedback loop to provide feedback from an output of the differentialinput stage amplifier to an input tail current of the differential inputstage amplifier, wherein the feedback loop enables class AB operation ofthe differential input stage amplifier; and at least one gain reduceroperatively coupled to the feedback loop to reduce a gain of thefeedback loop, wherein the gain reducer has a resistance value thatvaries inversely proportionally to a loop current in the feedback loopto reduce the gain of the feedback loop as the loop current increases,wherein the circuit is configured as a low dropout linear regulatorwhere the differential input voltage includes a reference voltagecomponent and a feedback component supplied from an output of theregulator.
 2. The circuit of claim 1, wherein the feedback loop includesa negative feedback loop to provide feedback from the output of thedifferential input stage amplifier to the input tail current of thedifferential input stage amplifier.
 3. The circuit of claim 2, whereinthe feedback loop includes a positive feedback loop to provide feedbackfrom the output of the differential input stage amplifier to the inputtail current of the differential input stage amplifier, wherein thepositive feedback loop reduces an amount of the differential inputvoltage to initiate nonlinear operations of the differential input stageamplifier with respect to changes in the differential input voltage. 4.The circuit of claim 3, wherein a gain of the positive feedback loop isset less than a gain of the negative feedback loop.
 5. The circuit ofclaim 4, wherein the gain of the positive feedback loop is set to aboutone fourth the gain of the negative feedback loop and boosts tailcurrent in the differential input stage amplifier by about 1000 timeswith a differential input change that includes a range of about 100 mVto about 120 mV.
 6. The circuit of claim 1, wherein the gain reducer isan active device that has a property of resistance specified as 1/gm,where the resistance varies inversely with an amount of current flowingthrough the gain reducer.
 7. The circuit of claim 6, wherein the gainreducer is a diode or a transistor configured as a diode that has aresistance of 1/gm.
 8. The circuit of claim 1, further comprising a passdevice that is driven from an output of the differential input stageamplifier to regulate the output of the regulator.
 9. The circuit ofclaim 1, further comprising an output driver stage to amplify the outputvoltage from the differential input stage amplifier and generate anamplified output voltage to provide an operational amplifier output. 10.A circuit comprising: a differential input stage amplifier that receivesa differential input voltage and generates an output voltage based on adifference in the differential input voltage; a negative feedback loopto provide feedback from an output of the differential input stageamplifier to an input tail current of the differential input stageamplifier, wherein the negative feedback loop enables class AB operationof the differential input stage amplifier; a positive feedback loop toprovide feedback from an output of the differential input stageamplifier to the input tail current of the differential input stageamplifier, wherein the positive feedback loop reduces an amount of thedifferential input voltage to initiate nonlinear operations of thedifferential input stage amplifier with respect to changes in thedifferential input voltage; and at least one gain reducer operativelycoupled to the negative feedback loop or the positive feedback loop toreduce a gain of the negative feedback loop or the positive feedbackloop respectively, wherein the gain reducer has a resistance value thatvaries inversely proportionally to a loop current in the negativefeedback loop or the positive feedback loop to reduce the gain of therespective loop as the loop current increases.
 11. The circuit of claim10, wherein the gain of the positive feedback loop is set less than thegain of the negative feedback loop.
 12. The circuit of claim 10, whereinthe gain of the positive feedback loop is set to about one fourth thegain of the negative feedback loop and boosts a tail current in thedifferential input stage amplifier by about 1000 times with adifferential input change that includes a range of about 100 mV to about120 mV.
 13. The circuit of claim 10, wherein the gain reducer is anactive device that has a property of resistance specified as 1/gm, wherethe resistance varies inversely with an amount of current flowingthrough the gain reducer.
 14. The circuit of claim 10, wherein thecircuit is configured as low dropout regulator circuit or as anoperational amplifier.
 15. A circuit comprising: a differential inputstage amplifier that receives a differential input voltage and generatesan output voltage based on a difference in the differential inputvoltage; a negative feedback loop to provide feedback from an output ofthe differential input stage amplifier to an input tail current of thedifferential input stage amplifier, wherein the negative feedback loopenables class AB operation of the differential input stage amplifier; apositive feedback loop to provide feedback from an output of thedifferential input stage amplifier to the input tail current of thedifferential input stage amplifier, wherein the positive feedback loopreduces an amount of the differential input voltage to initiatenonlinear operations of the differential input stage amplifier withrespect to changes in the differential input voltage; at least one gainreducer operatively coupled to the negative feedback loop and thepositive feedback loop to reduce a gain of the negative feedback loopand the positive feedback loop, wherein the gain reducer has aresistance value that varies inversely proportionally to a loop currentin the negative feedback loop and the positive feedback loop to reducethe gain of the respective loop as the loop current increases; and anoutput driver stage to amplify the output voltage from the differentialinput stage amplifier and generate an amplified output voltage toprovide an operational amplifier output.
 16. The circuit of claim 15,wherein the gain of the positive feedback loop is set less than the gainof the negative feedback loop.
 17. The circuit of claim 15, wherein thegain of the positive feedback loop is set to about one fourth the gainof the negative feedback loop and boosts a tail current in thedifferential input stage amplifier by about 1000 times with adifferential input change that includes the range of about 100 mV toabout 120 mV.
 18. The circuit of claim 15, wherein the gain reducer isan active device that has a property of resistance specified as 1/gm,where the resistance varies inversely with an amount of current flowingthrough the gain reducer.